1. Field of the Invention
The present invention relates to an electrical rotating machine, and more specifically, to an improved electrical rotating machine in which a local rise of temperature in a stator core is reduced.
2. Description of the Related Art
Electrical steel sheets are die-cut into substantially sector shapes as shown in FIG. 2 and arranged in a circumferential direction to form circular sheets which are stacked in an axial direction to form a stator core of a large size electrical rotating machine.
In FIG. 2, a sheet segment in a substantially sector shape includes teeth 1 between which there is provided a slot 3 where a coil is inserted, and a yoke 2 which forms a peripheral part of the sheet segment.
As shown in FIG. 3, several to several dozens of electrical steel sheets area stacked to form a block 4 and a cooling duct 5 is provided between blocks 4 to cool down an inside of a large size electrical rotating machine. The coil 6 and rotator 7 illustrate the relative position of the teeth region.
Among electrical steel sheets used for a stator core in an electrical rotating machine are non-grain oriented steel sheets in which a direction of easy magnetization is random and grain oriented steel sheets in which a direction of easy magnetization is aligned with a rolling direction of a steel band.
To reduce iron losses, grain oriented steel sheets are used as steel sheets for an iron core and circumferential directions of segment sheets are aligned with a rolling direction of a steel band as shown in FIG. 2 so that magnetic flux in a circumferential direction of a yoke 2 passes in a direction of easy magnetization. Moreover, circumferential directions of sheet segments of non-grain oriented steel sheets are also aligned with a rolling direction of a steel band since magnetic flux tends to pass more easily in the rolling direction of the steel band.
In teeth of end regions of a stator core, local overheating of the stator core becomes a problem since magnetic flux comes in an axial direction as well as radial directions. Thus, in a stator core using grain oriented steel sheets, magnetic flux cannot easily pass compared with in a stator core using non-grain oriented steel sheets, in the teeth 1 where magnetic flux passes mainly in the radial directions, since a direction of easy magnetization is aligned with a circumferential direction, and accordingly high heat is generated caused by iron losses. Local overheating in a stator core may damage insulation and form short circuits, causing a burnout of the stator core.
Therefore, as a solution for preventing local overheating in teeth of end regions of a stator core, a slit 8 is provided to each of teeth 1 as shown in FIG. 4 to narrow a flow path of eddy current in each of the teeth 1 and reduce eddy current loss caused by axial magnetic flux. Moreover, blocks 4 in each of end regions of a stator core are formed in a stepped taper shape as shown in FIG. 5 to prevent magnetic flux from locally concentrating.
However, losses caused by radial magnetic flux also greatly contribute to heating upon high densities of radial magnetic flux in teeth. Therefore, it is effective to reduce losses caused by radial magnetic flux to prevent local overheating.
The prior art described above is disclosed in Japanese Patent Publications JP 2000-50539A and JP 63-217940A and The Institute of Electrical Engineers of Japan; “Improvements and Problems for Use of Silicone Steel Sheets”, Technical Report of the Institute of Electrical Engineers of Japan, Part II, No. 85, in 1979.